The present application relates to a cathode active material which includes a complex oxide including lithium (Li) and cobalt (Co), a method of manufacturing the cathode active material, and a battery using the cathode active material.
In recent years, according to the widespread use of portable devices such as video cameras and laptop computers, small-sized high-capacity secondary batteries have been in increasing demand. Currently-used secondary batteries include nickel-cadmium batteries using an alkali electrolytic solution; however, in the nickel-cadmium batteries, the battery voltage is as low as 1.2 V, so it is difficult to improve the energy density. Therefore, a so-called lithium metal secondary battery using lithium metal having specific gravity of 0.534 which is the lightest among the simple substances of solids, an extremely base potential, and the highest current capacity per unit mass among metal anode materials has been developed. However, in the lithium metal secondary battery, as the lithium metal secondary battery is charged and discharged, lithium is grown into a dendrite form in an anode, thereby there are some issues such as a decline in cycle characteristics and the occurrence of an internal short circuit due to lithium penetrating through a separator. Therefore, a secondary battery using a carbon material such as coke for an anode and repeating charge and discharge by inserting and extracting alkali metal ions has been developed, and the deterioration of the anode due to charge and discharge is reduced (for example, refer to Japanese Unexamined Patent Application Publication No. S62-90863).
On the other hand, as a cathode active material capable of obtaining a battery voltage of approximately 4 V, transition metal chalcogenides including alkali metal are known. Among them, a complex oxide such as lithium cobalt oxide or lithium nickel oxide holds great promise in terms of a high potential, stability and long life, and in particular, lithium cobalt oxide has a high potential, so it is expected that the energy density will be improved by increasing a charge voltage.
However, when the charge voltage is increased, an oxidation atmosphere near a cathode is enhanced, thereby an electrolyte is easily deteriorated due to oxidative decomposition, or cobalt is easily eluted from the cathode. As a result, charge-discharge efficiency declines, and cycle characteristics or high temperature characteristics decline, so it is difficult to increase the charge voltage.
As a related-art technique for improving the stability of the cathode active material, a technique of forming a coating layer on surfaces of complex oxide particles is known. Moreover, as a method of forming a coating layer, a technique of coating surfaces of complex oxide particles with a metal hydroxide, and then performing a heat treatment is known (for example, refer to Japanese Unexamined Patent Application Publication Nos. H9-265985 and H11-71114).
However, when complex oxide particles are covered with a metal hydroxide, and then heated, particles are easily sintered, thereby particles are easily bonded. As a result, when an electrical conductor or the like is mixed at the time of forming a cathode, a bonded part and particles are ruptured or cracked to cause the separation of a coating layer or expose fracture surfaces of the particles. Such fracture surfaces have very high activity, compared to a surface formed at the time of firing, so the deteriorative reaction of an electrolytic solution and a cathode active material easily occurs.